Abstract: To study the diffusion law of sulfate anion in cemented gangue backfill material in mine water, take out the test samples of different ages with different concentrations of sulfate, the sulfate anion content was measured by layered sampling method, and chemical analysis method, and the diffusion coefficient was calculated according to the second Fick’s law. The results showed that as the concentration of external sulfate increases, the sulfate anion content in the specimen increases. The diffusion coefficient of sulfate ion is increased; the presence of bicarbonate ions(HCO^-₃) and magnesium ions(Mg²⁺) affects the diffusion rate of sulfate anion and bicarbonate anion has more significant effects than magnesium ion; sulfate anion reacts with cemented gangue backfill material producing the ettringite and gypsum, blocking pores. Change the diffusion path of ions, in the early stage, the ion diffusion rate is rapidly reduced. As the reaction time increases, the reaction product increases, and the crack is expanded, so that the ion diffusion rate is slowly lowered.